1. Field of the Invention
The invention relates in general to an apparatus for converting a digital signal to a corresponding analog signal and a method thereof, and more particularly to an apparatus for converting a digital pixel signal to a corresponding analog voltage signal for a liquid crystal display and a method thereof.
2. Description of the Related Art
Featuring the favorable advantages of thinness, lightness, and generating low radiation, liquid crystal displays (LCDs) have been widely used. The LCD panel includes a number of pixels, and the light transmittance of each pixel is determined by the voltage difference between the upper plate voltage and the lower plate voltage. The light transmittance of every pixel is typically non-linear with respect to the voltage applied across the pixel. Thus, gamma correction is performed to reduce color distortion by adjusting the lightness or darkness of pixels of the LCD panel.
FIG. 1 shows the gamma relation between the gamma voltage applied to a pixel and the luminance of the pixel. The X-axis represents the gamma voltage applied to the pixel, that is, the voltage difference between the upper plate and the lower plate voltages and the Y-axis represents the light transmittance of the corresponding pixel (T). When the magnitude of the upper plate voltage is fixed at a value, for example, Vcom, the voltage difference between the upper plate voltage and lower plate voltage is determined by the magnitude of the lower plate voltage. The corresponding relation between the lower plate voltage and the light transmittance of the pixel is nonlinear, as shown by the gamma curve in FIG. 1.
In addition, the gamma curve is symmetric with respect to the voltage of Vcom because the light transmittance of the pixel relates to the voltage across the pixel and is independent of the polarities of the voltages applied to the pixel. If two gamma voltages with the same magnitude but opposite polarities, for example, a positive gamma voltage Va and a negative gamma voltage Vb, are individually applied to the pixel, the light transmittance of the pixel is identical (TO). In other words, if the upper plates of two pixels are supplied with the voltage Vcom and the lower plate of one pixel is supplied with the voltage Va and the lower plate of another pixel is supplied with the voltage Vb, the luminance of the two pixels will be identical.
The liquid crystal molecules may deteriorate if a pixel of the LCD panel is supplied with voltages in the same polarity continually. Hence, the liquid crystal molecules can be protected by applying voltages in opposite polarity alternately across the upper and lower plates for each pixel. In other words, when a pixel has to emit at a luminance continuously, voltages in opposite polarities can be applied across the upper and the lower plates alternately by changing two different voltages across the upper and lower plates for the pixel alternately. In this way, deterioration of the pixel can be avoided.
FIG. 2 shows a block diagram of a nonlinear digital-to-analog converter (D/A converter) 202. The driving circuit of the liquid crystal display includes a nonlinear digital-to-analog converter 202 for converting the digital pixel signal (DATA) to the corresponding analog gamma voltage signal (OUT). Since the relation between the luminance of the pixel and the gamma voltage is not linear, the corresponding relation between digital pixel signal (DATA) and the analog gamma voltage signal (OUT) is determined according to the gamma curve. This process is called gamma correction. The corresponding relation between the digital pixel signal (DATA) and the luminance of the pixel is then approximated as linear by executing the gamma correction using the nonlinear digital-to-analog converter 202.
FIG. 3 shows a gamma curve, which is for use in the nonlinear digital-to-analog converter to perform gamma correction. The X-axis represents the data value of the digital pixel signal and the Y-axis represents the gamma voltage signal. The gamma curve shown in FIG. 3 includes a positive polarity gamma curve 404 and a negative polarity gamma curve 402. Each digital pixel signal corresponds to a positive polarity gamma voltage signal on the positive polarity gamma curve 404 or a negative polarity gamma voltage signal on the negative polarity gamma curve 402. The points A, B, C, D and E chosen from the positive polarity gamma curve 404 and the points A′, B′, C′, D′ and E′ chosen from the negative polarity gamma curve 402 are specific reference points. According to the gamma curve shown in FIG. 3, each reference point corresponds to a reference gamma voltage signal (GMV) and a reference digital pixel signal. When performing the gamma correction, the nonlinear digital-to-analog converter 202 converts each digital pixel signal to the corresponding gamma voltage signal by interpolation according to the relationship between the reference gamma voltage signal (GMV) and the corresponding reference digital pixel signal.
FIG. 4 shows a conventional apparatus for outputting the gamma voltage signals according to the reference gamma voltage signals, wherein the conventional apparatus for outputting the gamma voltage signals includes two strings of resistors. Each resistor string includes 255 resistors (R0˜R254), five input nodes (V0˜V4, V5˜V9) for receiving the reference gamma voltage signals, and 256 output nodes for outputting the gray level voltage signals. When the gamma correction is executed, the gamma output voltage signal corresponding to the digital pixel signal can be determined according to the gray level voltage signals.
FIG. 5 shows the diagram of the pixel P(N,M). The driving circuit of the pixel P(N,M) includes a thin film transistor T(N,M) and a pixel capacitor C(N,M). The gate electrode of the transistor T(N,M) is coupled to the scan line (SN) SN; the source electrode of the transistor T(N,M) is coupled to the data line (DM) DM; and the drain electrode of the transistor T(N,M) is coupled to the pixel capacitor C(N,M). When the transistor T(N,M) is turned ON through enabling the scan line SN, the gamma voltage output signal is delivered to the pixel capacitor C(N,M) through the data line DM and the transistor T(N,M). The luminance of the pixel P(N,M) can be determined by data value of the gamma voltage output signal.
In a color LCD, a picture frame is displayed based on a pixel element, called a color pixel or pixel simply, including three sub-pixels for displaying primary colors, that is, red, green, and blue. The three sub-pixels of a color pixel are supplied with separate gamma voltage signals outputted by the driving circuit of the color LCD after gamma correction. The pixel can thus display different colors by changing the brightness of the three sub-pixels individually.
FIG. 6 shows three different gamma curves, marked “R”, “G”, and “B”, for the primary colors, red, green, and blue, respectively. According to the “R”, “G”, and “B” gamma curves, the gamma voltages corresponding to the maximum luminance of the sub-pixels are VRM, VBM, and VGM for red, blue, and green respectively. The magnitude of VBM is smaller than that of VGM, and VGM is smaller than VRM (VBM<VGM<VRM). The nonlinear digital-to-analog converter conventionally predetermines the maximum magnitude of the gamma voltage signal to be VBM for gamma correction. Based on this magnitude of VBM,all other gamma voltage signals corresponding to the digital pixel signals are determined. Therefore, the relation between digital pixel signals and the corresponding gamma voltage signals is fixed and independent of the display color of the pixel corresponding to the digital pixel signal. Unfortunately, this conventional gamma correction method disadvantageously causes the luminance of a pixel being unable to reach its maximum value when the display color of the pixel is red or green, because the maximum magnitude of the gamma voltage signal is set to VBM while VBM is smaller than that of VGM and VGM is smaller than VRM (VBM<VGM<VRM). In this way, optimum display quality of the LCD panel becomes unachievable and the display performance would be degraded.